First, investigate the failed units. These frequently can be reports of attempts of faileed SSH connections, or dropped sessions from legitimate users.

Then clear them:

sudo systemctl reset-failed

This happened to me as well. Join me on my journey to find an answer, below

This happens when some systemd unit fails on the base system. (This has nothing to do with docker, or docker images.)

Presumably, the sshd server is failing, which may or may not be related to a dropped connection.

You can use these commands to see more information:

$ systemctl --failed
$ systemd status sshd@16493-188.166.209.155:22-222.186.15.79:2573.service

For reference, I'm getting the following error:

Failed to run 'start' task: Transport endpoint is not connected

Here is my output from list-units, which helped me figure this out, finally:

$ systemctl list-units | grep sshd
  sshd-keygen.service                                                           loaded active exited    Generate sshd host keys
  sshd@1296-104.236.58.186:22-137.22.169.192:43537.service                       loaded active running   OpenSSH per-connection server daemon (98.22.169.192:43537)
● sshd@532-104.236.58.186:22-277.186.58.136:1283.service                        loaded failed failed    OpenSSH per-connection server daemon (222.186.58.136:1283)
  system-sshd.slice                                                             loaded active active    system-sshd.slice
  sshd.socket                                                                   loaded active listening OpenSSH Server Socket

It looks like sshd.socket is where the sshd daemon listens for new connections. The sshd@ lines report one failure and one success (for me). These appear to take the form of

sshd@1234-<server-IP>:<server-port>-<remote-IP>:<remote-port>.service

This explains what happened: I tried logging in from work the other day, and did not have the correct ssh key. So, for me at least, this represents a failed login. While that represents a security crisis averted — and I'm grateful that CoreOS brought this to my attention — I haven't figured how to clear it out yet.

• CoreOS brief explanation
• DigitalOcean systemd tutorial

Good luck with your new web service!

{"reason":"Nothing here."}

It means somebody (not necessarily yourself, perhaps a bad guy) tried to login via ssh, and failed.

To protect the host from compromise / resource exhaustion, you may want to change port & disable password authentication in sshd_config. Many script kids stops trying when they find that password auth is not available.